1. Field of the Invention
The present invention relates generally to traveling wave tubes. More particularly, a traveling wave tube having an electron gun using a multilayer carbon-based field emitting cathode is provided.
2. Description of Related Art
There are two basic geometries of field emission electron devices. The first geometry uses arrays of electron emitting tips. These devices are fabricated using complex photolithographic techniques to form emitting tips that are typically one to several micrometers in height and that have an extremely small radius of curvature. The tips are commonly composed of silicon, molybdenum, tungsten, and/or other refractory metals. Prior art further suggests that microtips microtips can be fabricated from diamond of a specific crystal orientation or that non-carbon microtips can be coated with diamond or a diamond-like carbon to enhance their performance. (U.S. Pat. No. 5,199,918) Also, a class of microtips based on the fabrication of thin wires or whiskers of various materials, including carbon has been described (xe2x80x9cField Emission from Nanotube Bundle Emitters at Low Fields,xe2x80x9d Q. Wang et al, App. Phys. Lett. 70, [24], pp. 3308 (1997)).
The second prior art method of fabricating a field emission device is based upon a low or negative electron affinity surface usually composed of diamond and/or diamond-like carbon (U.S. Pat. No. 5,341,063; U.S. Pat. No. 5,602,439). These devices may be formed into tips or they may be flat. Other wide bandgap materials (mainly Group III nitrides) have also been suggested as field emission devices due to their negative electron affinity properties.
In the first method, complex lithographic and/or other fabrication techniques are needed to fabricate the tips. Additionally, tips made from non-diamond materials have short functional lifetimes due to resistive heating of the tips and poisoning of the tips due to back-sputtering from the anode. Diamond-based microtips solve those two problems to some degree but typically require many negative electron affinity surfaces in order to function properly.
The second method requires a low or negative electron affinity surface for the devices to work. Additionally, the prior art suggests that an improved diamond or diamond-like emitter can be fabricated by allowing for screw dislocations or other defects in the carbon lattice. (U.S. Pat. No. 5,619,092). Diamond-based materials having current densities of 10 A/cm2 have recently been described. (T. Habermann, J. Vac. Sci. Tech. B16, p. 693 (1998)). These devices are fabricated on and remain on a substrate.
A very recent paper describes gated and ungated diamond microtips. (D. E. Patterson et al, Mat. Res. Soc. Symp. Proc. 509 (1998)). Some ungated emitters were reported to allow electrical current of 7.5 microamps per tip. The process variables used to form the emitters were not discussed. If tips could be formed at a density of 2.5xc3x97107 tips/cm2, it was calculated that the current density could be as high as 175 A/cm2, assuming that all the tips emit and that they emit uniformly.
Different characteristics of field emitters are required for different devices. For some devices, such as flat panel displays, sensors and high-frequency devices, emission at low electric fields is particularly desirable to minimize power requirements. For other devices, higher threshold electric fields for emission are tolerable, but high currents are required. High currents are particularly needed for some applications of electron guns, in amplifiers and in some power supplies, such as magnetrons and klystrons.
Accordingly, a need exists for an improved carbon-based electron emitter that does not involve the fabrication of complex, micrometer-sized (or smaller) structures with tips or structures that require certain crystallographic orientations or specific defects in order to function properly. Additionally, these emitters should provide high levels of emission current with moderate electric fields. Preferably, the emitters should have a thickness sufficient for the emitter material to have mechanical strength in the absence of a substrate, making free-standing electron sources that are suitable for use in a variety of electronic apparatus, including traveling wave tubes.
In accordance with the present invention, a high current density carbon-based electron emitter is formed by chemical or physical vapor deposition of carbon to form a bulk structure having two layers of carbon-based material. The bulk material or body grown in this manner is believed to provide a high thermal conductivity matrix surrounding conductive carbon channels, so that the resistive heating in the conductive channels, even at high currents, can be dissipated from the channels. Electrons are ultimately emitted from the carbon surface by means of field emission from the conductive channels. In addition, the emitting layer is in direct contact with a thicker layer having very high thermal conductivity, so that heat can be transferred from the emitting layer at a rate to avoid excessive temperature and failure of the emitting layer.
The carbon-based body is grown by placing a substrate in a reactor, lowering the pressure in the reactor and supplying a mixture of gases that includes hydrogen and a carbon-containing gas such as methane at a concentration from 8 to 13 percent to the reactor. High energy is supplied to the gases near the substrate. The energy may be supplied by several methods, such as a microwave or RF plasma. The substrate is brought to a selected range of temperatures via active heating or cooling of the substrate stage within the reactor. After a layer has grown to a thickness of a few micrometers the concentration of methane is decreased and a second, much thicker layer is grown. Then the substrate is removed, leaving a stand-alone body of carbon-based material having two layers. Each layer has a preferred range of electrical resistivity. An electrode is placed on the surface of the thicker layer. Electron emission is stable with high current density from the surface of the thinner layer. This surface may be flat or may be structured. A structured surface on the carbon-based body is achieved by structuring the surface of the substrate before the emission layer is grown.
Devices based on high current density electron emission from the carbon-based body are provided. These include electron guns and cathode ray tubes containing the electron guns, amplifiers and traveling wave tubes.